Despite two bomb scares already investigated by Irish police, Queen Elizabeth II is continuing on with her historic visit to the Republic of Ireland today, the first by a British monarch since the country acquired its independence from the United Kingdom in 1922.

The Queen, 85, landed in Dublin today after police there discovered a "viable" explosive device aboard a bus headed to Dublin overnight. Police removed the explosives after a tip came in concerning a "suspicious device," according to a police spokesperson. A second suspected device, abandoned near a train station in west Dublin, turned out to be a hoax.

Police have not said who may have been behind the threats, but Irish security forces have mounted the most extensive security operation in the country's history ahead of the Queen's visit, anticipating a possible incident. Three bomb threats were called into London's Scotland Yard Monday by Irish Republican dissidents, but none mentioned the Queen's four-day tour of the Republic of Ireland.

Those threats were seen by authorities as the latest in an aggressive campaign by Irish terrorists that has so far confined itself largely to attacks on police and security services in areas inside Northern Ireland that include Belfast, Omagh and Londonderry.

According to senior British security officials, a small group of resurgent Irish terrorist has been responsible for more than 40 significant terror attacks over the last year. The British officials in charge of countering domestic terrorism, MI5, have increased surveillance of suspected Irish terror leaders and shifted resources from al Qaeda to the Irish groups. Largely unnoticed outside of the United Kingdom, splinter groups of Irish Republican militants have increasingly ignored the peace agreement reached 13 years ago that was meant to end IRA violence.

But to the British Foreign Ministry, the fact that the Queen's visit could happen at all is a sign anti-British sentiment in Ireland -- punctuated for years by a violent opposition -- has improved to unprecedented levels.

"The Queen's visit demonstrates the closeness of our relationship with our nearest neighbour, and how far this has progressed over the last decade -- in part due to the success of the Northern Ireland Peace Process," says a post on the ministry's website. "It is also an opportunity to celebrate the strength of our long-standing economic, political and cultural ties which now bind us together closer than ever before."

The Queen, accompanied on her trip by the Duke of Edinburgh, was greeted upon landing by Irish President Mary McAleese, after which she was scheduled to take part in a ceremony at Ireland's Garden of Remembrance, which is "dedicated to the memory of all those who gave their lives in the cause of Irish freedom," before a state dinner at the Dublin Castle. She is traveling in a bullet and bomb proof Range Rover, according to a report by The Associated Press.